1. Field of the Invention
This invention is in the field of methods and apparatus for the application of coatings to glass containers. More particularly, the present invention is in the field of methods and devices for control of the application of coatings of varying thicknesses to bottles, jars and the like.
2. Description of the Prior Art
The utility of glass bottles and jars has been broadened by surface coating to decrease abrasion and breakage as taught by Carl, et al., U.S. Pat. No. 3,323,889; Gatchet, et al., U.S. Pat. No. 3,516,811; Scholes, et al., U.S. Pat. No. 3,819,404; Hofmann, et al., U.S. Pat. No. 4,431,692; Lindner, et al., U.S. Pat. No. 4,668,268, and others. Gatchet observed the utility of avoiding all coating on the closure region of the container, known in the art as the "finish". In U.S. Pat. No. 4,431,692, Hofmann taught maintaining the finish out of contact with the treatment gas. Several of the prior workers in this field recognized the existence of non-linear currents in the coating precursor stream, including omnidirectional turbulent currents and upwardly-moving convention currents.
While well-adjusted coating hoods of the prior art can provide an acceptably high body coating with acceptably low finish coating for a period of time, we have identified destabilizing effects not completely controlled by the prior art. When a well-adjusted coating hood is converted to the coating of a different container, not all of the careful adjustments survive, nor do adjustments for a given style of container always stay within acceptable ranges for the duration of a coating run.
Those skilled in the art will realize that turbulent and connection currents, and the current caused by impingement of the coating current upon the moving glass containers, will vary when production is changed to ware of differing mass, dimension, shape, production speed, spacing on the conveyor belt, or any combination of those and other production variables. Even with sufficient time and adequate instrumentation, it has been found virtually impossible to readjust the balance of finish-protection and coating streams quickly and economically. But once that balance has been reestablished, it can shift, as when a single section of the forming line comes on or goes off stream, or any of the variables mentioned above changes again. Additionally, the balance will drift, as coating vapors slowly pyrolyze on the coating tunnel, building up to thickness levels which cause disruption of the normal flow of the process streams.
In the teaching of the prior art, a limiting factor arises when the finish-protection stream must be increased, and this factor of itself causes unintended currents, including deflection at the point of impingement upon the finish, swirling of the adjacent air mass, and induced drafts. FIGS. 1 through 6 show the tendency of these unintended currents to bring coating vapors into the vicinity of the finish, in the operation of coating apparatus of the prior art. In each of the figures discussed here, the representation is simplified for purposes of illustrating the principal differences between the prior art and the present invention.
FIG. 1, after Gatchet, shows the idealized, essentially horizontal flow of the coating stream 11 across the path of moving bottles 212.
FIG. 2, from the disclosure of Frank, U.S. Pat. No. 3,623,854, shows the use of convection currents 21 caused by the hot glass container 212 heating the ambient air to effect upward distribution of coating vapors from jets 22 positioned below the center of the bottle, and his use of a jet 23 of clean air to prevent coating of the finish by the convection currents.
FIG. 3 shows deflection of the horizontal coating stream 11 of Gatchet by impingement upon the glass surface 31. Those skilled in the art will understand that the convective upward deflection 32 as taught by Frank is also present, as are multiple turbulent currents 33 caused by interaction of the various gaseous streams with each other, and with the moving ware 212.
FIG. 4 shows the induced draft 41, carried along by Frank's jet of clean air 23 and combining therewith, because the pressure in jet 23 is reduced by its relatively higher velocity. Air 42, replacing the air taken away by induced draft 41, rises through the coating stream because of both the lower pressure above and the convection current 43 created by the heat from bottle 212.
FIG. 5 shows an advanced embodiment of the prior art as disclosed by Lindner et al. in U.S. Pat. No. 4,668,268, wherein a large volume of low-pressure air from finish-protection jets 51 floods the upper portion of the center section 52, preventing most of the coating vapors 53 from reaching the finish region 54 of bottle 212.
FIG. 6 shows the center section 52 of FIG. 5 after two months of substantially continuous operation. The combination of high-energy convection and impingement currents, plus induced drafts, turbulence, and diffusion into the low-energy air mass above, has gradually deposited coating 61 on the inner walls 62 of center section 52 until the deposit is several millimeters (mm.) thick, and has begun to clog finish-protection jets 51, leading to turbulent currents 63 in the process, an even more rapid deposit formation, complete blockage of the finish-protection jets 51, and resultant undesirable heavy coating of the finish 54.
It is known in the prior art to direct a finish-protecting stream of clean air downward toward the finish, from stream-directing means in the roof of the coating tunnel. Unfortunately, if the downward stream has the velocity required to stop the upward components of the turbulent, impingement, and convection currents, it can seriously dilute and displace the coating-precursor current in the shoulder region, where the surface protection provided by an adequate coating level is critical. The operator of the coating system is then faced with the choice of having unacceptably thick coating on the finish, or unacceptably low coating in the shoulder region, on an unacceptably high percentage of the processing run.